1. Field of the Invention:
The invention relates to the use of polyaddition products containing hydroxyl and uretdione groups to prepare solvent-free polyurethanes which do not give off elimination products, and to the one-component, heat-curable polyurethane (PU) adhesives prepared accordingly.
2. Discussion of the Background Art:
Polyurethane adhesives are generally notable for very good properties of adhesion to surfaces of a very wide range of materials, for resistance to solvents, plasticizers, fats, oils and water, and for the high flexibility of the adhesive films even at low temperatures. Owing to the great variety of polyurethane chemistry, the literature--see e.g. G. Habenicht, Kleben -Grundlagen, Technologie, Anwendungen (Bonding--principles, technology, applications) , Springer Verlag, Berlin, Heidelberg, New York, Tokyo, 1986--recognizes a number of different types of polyurethane adhesives. A distinction is made between one-component ("one-pack") and two-component ("two-pack") systems. Two-pack polyurethane adhesives are essentially characterized by polyisocyanates as hardeners and by predominantly oligomeric diols and/or polyols as resin. They have the advantage of presenting no great problems in terms of shelf life and that by a skillful choice and targeted reactivity of the monomers it is possible to formulate systems having different pot lives and adhesive-film-determining properties, for example strength, elasticity and resistance to chemicals. Owing to polyaddition reactions, these adhesives do not release any elimination products in the course of crosslinking.
The technical effort required when processing two-pack systems in terms of precise dosing and mixing of the components has led to the development of one-pack systems, which are easier to process. These contain isocyanato prepolymers which are prepared from polyols of relatively high molecular mass with a stoichiometric excess of polyisocyanate. Full curing takes place in most cases through moisture-induced crosslinking. For this reason, sufficient atmospheric humidity (at least 40% rel. humidity) in the processing areas is required. Consequently, these adhesives are of only limited usefulness for bonds where the parts to be joined are metallic or otherwise moisture-impermeable. Owing to the exclusion of moisture, the packaging of these one-pack adhesives during transportation and storage is critical.
A further development comprises the after-crosslinking polyurethane hot-melt adhesives (reactive hot melts) which following application from the melt provide, after cooling (physically setting), a spontaneous initial strength and subsequently react, under the action of further ambient atmospheric humidity on the remaining reactive isocyanate groups, to form the adhesive-film polymer.
In the case of the crosslinking reaction by means of moisture, the formation of carbon dioxide as a result of the breakdown of the unstable carbamic acid derivatives may be critical, especially when relatively thick adhesive films of relatively high viscosity are present and when the gas bubbles are enclosed in the adhesive film. Moreover, moisture-curing PU adhesives do not attain the strengths of two-pack PU adhesives.
An alternative to the moisture-curing systems is represented by one-pack heat-activatable PU adhesives. These consist of polyol resins and of polyisocyanates whose isocyanate groups are inhibited by so-called blocking agents. Elevated temperatures lead to cleavage of the hardener molecules, in the course of which the blocking agents are eliminated and the polymer is generated by polyurethane formation. Here too, a disadvantage is the formation of elimination products in the sense of the adverse effect this has on the adhesion properties. Moreover, the organic structure of some blocking agents renders them toxicologically unacceptable.
Polyurethanes are likewise employed as solvent-containing adhesive systems. In this case, a distinction may be made between physically setting and chemically reacting systems. Both systems possess high molecular mass hydroxy polyurethanes as polymers, while the chemically reacting systems additionally include a polyisocyanate as second component in the solvent system. In order to form the adhesive film, a process which may take place at normal or elevated temperature, it is necessary to remove the solvent.
However, the interests of environmental protection and adhesive technology make it desirable to provide adhesives which are free from emissions in the form of solvent or of elimination products, such as blocking agents.
The object on which the invention is based, therefore, was to develop new PU adhesives unhampered by the abovementioned disadvantages of the prior art; in other words, they should be one-component systems free from blocking agents and solvents and should be able to be crosslinked thermally to form thermosets. Furthermore, subsequent to curing, the PU adhesives should exhibit bonds having very good temperature and solvent resistance properties.
This object has been achieved in accordance with the disclosed invention. The text below describes the novel resin/hardener mixture in which the curing agents are polyaddition products containing hydroxyl and uretdione groups. The hydroxyl-containing resins are crosslinked both by a urethane reaction and by polymerization of the epoxide groups.